Light olefins, defined herein as ethylene and propylene, serve as feeds for the production of numerous chemicals. Olefins traditionally are produced by petroleum cracking. Because of the limited supply and/or the high cost of petroleum sources, the cost of producing olefins from petroleum sources has increased steadily.
Oxygenates such as alcohols, particularly methanol, dimethyl ether, and ethanol, are alternative feedstocks for the production of light olefins. In an oxygenate to olefin (OTO) reaction system, an oxygenate in an oxygenate-containing feedstock contacts a molecular sieve catalyst composition, preferably in a fast-fluidized reaction system, under conditions effective to convert at least a portion of the oxygenate to light olefins, which are yielded from the reaction system in a reaction effluent.
Typical OTO reaction processes occur in fast-fluidized reaction systems. An oxygenate-containing feedstock enters a lower region of a riser reactor through one or more conduits. Upon entry into the riser reactor the oxygenate-containing feedstock flows in an upward direction in the riser reactor while contacting a catalyst composition. As the oxygenate contacts the catalyst composition, the oxygenate in the oxygenate-containing feedstock is converted to light olefins. One problem associated with conventional fast-fluidized reaction systems is that the catalyst particles contained in the riser reactor may backflow into the one or more feed introduction conduits, particularly during start up and/or shutdown operations. This backflow of catalyst can interfere with the operation of the feed introduction conduits when operation of the riser reactor resumes. Special shut down procedures can be followed in order to reduce or eliminate backflow of particles. However, such procedures may not be possible when an emergency dictates the timeframe of a shutdown. The agglomeration of the catalyst particles that results from catalyst backflow may protract startup time and, in some situations, require time consuming and expensive disassembly of equipment before operations may resume.
U.S. Pat. No. 6,346,219 addresses a similar problem that occurs in fluidized catalytic cracking (FCC) units. Specifically, the '219 Patent is directed to a fluid feed distributor which improves particle and feed contacting by using a flow actuated plug to prevent backflow of fluidizable particles through a fluid feed outlet when fluid feed to a riser conduits ceases.
Japanese Patent No. 73020697B similarly provides a tap on a riser, which falls down in the case of no fluid feed thereby closing the riser and shutting down the connection of the feeding zone with the fluidization zone. An advantage of this invention is that it allows intimate contacting while protecting the backflow of catalyst particles into a fluid reactant feed pipe from the fluidization reaction vessel.
While various apparatus and processes have been provided in the prior art, the need exists for additional processes and devices for reducing the backflow of catalyst particles into a catalyst feed conduit, particularly in OTO reaction systems.